1. Field of the Invention
The invention relates to a method for preparing titanium dioxide particles, more particularly to a method for preparing titanium dioxide particles co-doped with nitrogen and fluorine.
2. Description of the Related Art
A photocatalyst is a catalyst which is capable of being excited by light energy to conduct a catalytic reaction. When the photocatalyst is irradiated by light, electrons in a valence band are excited to rise up to a conduction band, and corresponding holes are produced in the valence band, thereby forming electron/hole pairs. When the electrons and the holes react with water and oxygen, reactive free radicals, such as O, O−, O2−, O3−, OH−, etc., will be produced. Once the free radicals come into contact with organics, such as cell membranes of bacteria, the organics may be oxidized to produce water and carbon dioxide. Therefore, the cell membranes of the bacteria are destroyed, and a sterilization effect is achieved.
The compounds suitable for use as the photocatalyst include oxides, such as titanium dioxide, zinc oxide, niobium oxide, tungsten oxide, tin oxide, zirconium oxide, or the like, and sulfides, such as cadmium sulfide, zinc sulfide, or the like. Among them, titanium dioxide is a most popular photocatalyst used in the prevention of air pollution in view of its suitable energy gap, strong oxidation-reduction capability, high decomposition efficiency, non-toxic property, etc.
Generally, titanium dioxide has three types of crystal structures, i.e., anatase, rutile, and brookite. Among them, the anatase-type TiO2 is a primary material useful as the photocatalyst in view of its suitable energy gap of 3.2 eV and its superior optical activity.
However, the excitation of anatase to conduct a photocatalytic reaction requires an energy of more than 3.2 eV, which corresponds to a light having a wavelength smaller than 387 nm, i.e., an ultraviolet ray. Therefore, the application of anatase as the photocatalyst is limited.
It is desirable in the art to reduce the energy band of titanium dioxide to a level suitable for exciting titanium dioxide using visible light having a wavelength ranging, for example, from 400 nm to 700 nm (The corresponding energy gap ranges from 3.1 eV to 1.7 eV). It is known according to Di Li et al., “Fluorine-Doped TiO2 Powders Prepared by Spray Pyrolysis and Their Improved Photocatalytic Activiy for Decomposition of Gas-Phase Acetaldehyde,” Journal of Fluorine Chemistry, 2005, Vol. 126, pp. 69-77, that a visible light-driven photocatalysis can be obtained by doping titanium dioxide with fluorine to enhance surface acidity, to create oxygen vacancies, and to increase active sites.
It is known according to R. Asahi et al., “Visible-Light Photocatalysis in Nitrogen-Doped Titanium dioxides,” Science, Vol. 293, pp. 269-271, 13 Jul. 2001, that the substitutional doping of nitrogen (N) is the most effective among the substitutional doping of carbon (C), nitrogen (N), fluorine (F), phosphorous (P), or sulfur (S) for oxygen (O) in the anatase-type TiO2 crystal because N (p) states contribute to the band-gap narrowing by mixing with O (2p) states. Visible-light activity could be introduced in TiO2 by doping with N. The optical absorption spectra of TiO2 can be shifted to the range of visible light, and the required band gap can be lowered to 2.9 eV.
It is known according to Di Li et al., “Visible-Light-Driven N—F-Codoped TiO2 Photocatalysts. 1. Synthesis by Spray Pyrolysis and Surface Characterization,” Chem. Mater., 2005, 17, pp 2588-2595, and Di Li et al., “Visible-Light-Driven N—F-Codoped TiO2 Photocatalysts. 2. Optical Characterization, Photocatalysis, and Potential Application to Air Purification,” Chem. Mater., 2005, 17, pp. 2596-2602,that N—F-codoped TiO2 powders have a superior photocatalytic capability as compared to N-doped or F-doped TiO2 powders. The N—F-codoped TiO2 powders are synthesized by spray pyrolysis (SP) from a mixed aqueous solution containing TiCl4 (0.03M) and NH4F (0.20 M) as TiO2 and N/F precursors, respectively. A series of N—F-codoped TiO2 powders are prepared by changing the SP temperature. N and F concentrations of N—F-codoped TiO2 powders prepared at the SP temperature ranging from 500 to 1100° C. are shown in Table 1.
TABLE 1SPTotal-NTotal-Fsampletemperature (° C.)(at. %)(at. %)NFT-5005000.383.15NFT-6006001.192.80NFT-7007001.222.35NFT-8008000.831.90NFT-9009000.611.35NFT-100010000.521.01NFT-110011000.440.56
However, the N—F-codoped TiO2 powders suffer from the problems such as limited solid solubility and uneven distribution of N and F elements in the TiO2 powders due to the external doping of N and F elements with TiO2. Therefore, the photocatalytic capability of the N—F-codoped TiO2 powders is limited, and the oxidation-reduction effect is reduced. It is known according to Isamu Moriguchi et al., “Oriented Growth of Thin Films of Titanium Oxyfluoride at the Interface of an Air/Water Monolayer,” Chem. Commun., 2001, pp. 1344-1345, that, when an air/water monolayer of dioctadecyldimethylammonium bromide (DODMABr) is formed at 25° C. on the surface of a liquid-phase deposition (LPD) solution, which is a mixed aqueous solution of (NH4)2TiF6 and H3BO3 at 1≦B/Ti<1.5, oriented crystallites of NH4TiOF3 are produced and grown at a hydrophilic interface of the monolayer to yield a self-supporting thin film. The NH4TiOF3 crystallites can be converted into anatase-type TiO2 by air-calcination at 600° C.
However, as described above, the B/Ti molar ratio should be strictly limited to a relatively small range (i.e., 1≦B/Ti<1.5). Furthermore, a considerably large amount of NH4TiOF3 crystallites are deposited on the bottom of the reaction container, rather than at the monolayer. This means that the bonding strength between the NH4TiOF3 crystallites and the monolayer of DODMABr is considerably weak. Moreover, the monolayer of DODMABr may decompose during the air-calcination. Therefore, the aforesaid method is not suitable for industrial application.